It is well known that high frequency ultrasonic waves may be generated or received by piezoelectric or electrostrictive transducers operating in thickness vibration mode. Typically, one of two kinds of ultrasonic waves are used. The a first type is termed pulse and the second is called continuous wave. Because the spectrum of a pulse covers a broad frequency range, the former requires a broad band frequency response. The latter (i.e. continuous wave) can be of narrow frequency response. When resonance of a transducer is strong, the bandwidth is relatively narrow. Therefore, resonant transducers are generally not suitable for generation of a sharp pulse. When continuous wave is required, a resonant type transducer is suitable and the bandwidth can be narrow. Furthermore, a resonant type transducer can generate a high output power acoustic signal which is typically higher than that of non-resonant transducers. Also, resonant type transducers receive ultrasonic waves with a high degree of sensitivity and can generate a voltage output in response thereto.
There are various applications of high frequency ultrasound in continuous wave mode. Examples include (1) blood flow velocity measurement using Doppler shift, (2) liquid flow velocity measurement using phase differences between up-stream and down stream signals, (3) image formation using intensity of reflection from an object using a scanned focused beam, (4) distance measurement for varying reflector position from varying transducer impedance due to varying phase of reflection, and (5) ultrasound focused energy to ablate malignant organs such as prostate cancer or tumors (i.e. operations without cutting the skin).
In order to improve performance of an ultrasonic transducer, an impedance matching layer is often added at the front surface of the transducer. For instance, it is known in the art to have an impedance matching layer with a thickness of a quarter wavelength bonded at the front surface of a transducer. Also, conventional practice has implemented the theory that the best impedance matching is obtained at the condition of its acoustic impedance of geometrical mean value of the impedances of transducer material and radiation medium. Consistent with conventional practice, such a matching layer is obtained having an acoustic impedance value between a high impedance value associated with the transducer material, and a low impedance value corresponding to the radiation or propagation medium (typically, water).
Furthermore, it is generally known that a front matching layer added to a resonant type transducer makes the transducer wide band and higher output (receiving sensitivity). As evidenced through published articles and issued patents, such as U.S. Pat. Nos. 4,507,582, 4,211,948, and 4,672,591 suggesting that the best matching layer necessarily increases output or sensitivity of the transducer. This is because there is a common knowledge on electric power output, which is maximized when the load impedance is matched to the source impedance.
In the case of an ultrasonic transducer, the conventional impedance matching condition is the geometrical average of impedances of radiation medium and transducer material; where: EQU Z.sub.m =Z.sub.P +L Z.sub.R +L (Eq.1)
Z.sub.m =p.sub.m V.sub.m ; Matching layer impedance (p; density, V; velocity) PA1 Z.sub.R =p.sub.R V.sub.R ; Radiation medium impedance (p; density, V; velocity) PA1 Z.sub.p =p.sub.p V.sub.p ; Piezo material impedance (p; density, V; velocity) where Z.sub.p &gt;Z.sub.R and Z.sub.p &gt;Z.sub.m &gt;Z.sub.R, and the values of Z of these materials are determined in their natural state.
However, in accordance with the present invention as described herein, it has been determined that a resonant type transducer is different from a non-resonant transducer. In non-resonant transducers, the best matching structure is shown by Eq. (1) which operates to make the bandwidth narrower and output (sensitivity) higher. In resonant transducers, the conventional matching condition--satisfying Eq. (1); i.e. geometric average using matching layer with impedance greater than water and less than the determined high impedance of the piezo material transducer body--makes the bandwidth broader but the output (sensitivity) lower. Therefore, there is no advantage of the conventional matching layer for resonant transducers. The present invention proposes that the impedance of the matching layer should be much lower than the value provided by the conventional matching condition of Eq. (1) in order to improve output or receiver sensitivity.
Accordingly, while a matching condition wherein the matching layer impedance lies between a high impedance transducer material and a low impedance radiation medium (e.g. water) is acceptable for wideband matching, its application to high output or high sensitivity transducer applications (e.g. an acoustic surgical knife) is less than desirable. Therefore, a matching structure for coupling a transducer body to a radiation medium for providing a high output or high sensitivity ultrasound acoustic signal is greatly desired.